Scum-retardant carrier particles and compositions thereof



US. Cl. 252-62.1 10 Claims ABSTRACT OF THE DISCLOSURE Magneticallyresponsive carrier particles for use in magnetic-brush development ofelectrostatic images, com.- prising a core of ferromagnetic materialovercoated with a thin, continuous layer of a film-forming,alkali-soluble carboxylated polymer. The particles have a reducedtendency to cause scum formation on the surface of a photoconductiveelement.

This invention relates to electrophotography, and more particularly, tomagnetically attractable carrier particles useful in the magnetic-brushtype development of electrostatic latent images.

Electrophotographic imaging processes and techniques have beenextensively described in both the patent and other literature, forexample, US. Pat. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809;2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 andmany others. Generally, these processes have in common the steps ofemploying a normally insulating photoconductive element which isprepared to respond to imagew'ise exposure with electromagneticradiation by forming a latent electrostatic charge image. Theelectrostatic latent image is then rendered visible by a developmentstep in which the charged surface of the photoconductive element isbrought into contact with a suitable developer mix.

One method for applying the developer mix is by the well-known magneticbrush process. Such a process can utilize apparatus of the typedescribed, for example, in US. Pat. No. 3,003,462 and often comprises anonmagnetic rotatably mounted cylinder having fixed mag netic meansmounted inside. The cylinder is arranged to rotate so that part of thesurface is immersed in or otherwise contacted with a supply of developermix. The granular mass comprising the developer mix is magneticallyattracted to the surface of the cylinder. As the developer mix comeswithin the influence of the field generated by the magnetic means withinthe cylinder, the particles thereof arrange themselves in bristle-likeformations resembling a brush. The bristle formations of developer mixtend to conform to the lines of magnetic fiux, standing erect in thevicinity of the poles and lying substantially flat when said mix isoutside the environment of the magnetic poles. Within one revolution thecontinually rotating tube picks up developer mix from a supply sourceand returns part or all of this material to the supply. This mode ofoperation assures that fresh mix is always available to the copy sheetsurface at its point of contact with the brush. In a typical rotationalcycle, the roller performs the successive steps of developer-mix pickup,brush formation, brush contact with the photoconductive element, brushcollapse and finally mix release.

In magnetic-brush development of electrostatic images, the developer iscommonly a triboelectric mixture of fine toner powder comprised of dyedor pigmented thermoplastic resin with coarser carrier particles of asoft magnetic material such as ground chemical iron (iron filings),reduced iron oxide particles or the like. Such United States Patentcoarse carrier particles when used for repeated magnetic brushdevelopment of the same photoconductive element, often tend to scratchor otherwise abrade the photoconductive surface. Such scratchingultimately results in a poor quality image being produced.

Another undersirable quality of prior iron or ironalloy carrierparticles when used in developing compositions is the tendency of thedeveloper to deposit scum on the surface of a reusable photoconductiveelement. The continued contact of such developing compositions oftenresults in the deposition of a difficultly removable layer of surfacescum on the photoconductor. This layer of scum may comprise fines of thetoner powder and/or contaminant present on the surface of the carrierparticles. Regardless of the composition of such scum, it acts as anarea of irregular optical density which reduces the effectivesensitivity of the photoconductive layer. In addition, the effect ofthis scumming is magnified when there is abrasion of the photoconductoras mentioned above.

Accordingly, there is a need for iron carrier particles which reduce thetendency of magnetic-brush developer compositions to deposit scum on thesurface of reusable eleotrophotographic elements.

It is therefore an object of this invention to provide novelferromagnetic carrier particles having a reduced tendency to causescumrning of photoconductive layers.

It is another object of this invention to provide novel carrierparticles for use in magnetic-brush development which particles have areduced tendency to abrade the surface of the photoconductive element.

A further object of this invention is to providenew magneticallyresponsive carirer particles having high electrical surface resistance.

Still another object of this invention is to provide new developingcompositions for use in magnetic-brush development.

These and other objects and advantages are accomplished in accordancewith this invention by coating conventional ferromagnetic carrierparticles with thin layers of film-forming, alkali-soluble carboxylatedpolymers.

The carrier materials which can suitably be coated in accordance withthis invention include ferromagnetic materials such as iron in suchforms as reduced iron oxide bits, iron filings and the like; nickel;cobalt; as well as alloys containing nickel, iron and cobalt etc. Suchferromagnetic materials are used as a core in accordance with thisinvention over which is coated a carboxylated polymer. The core canconsist of a solid particle of ferromagnetic material or can be anonferromagnetic particle overcoated with ferromagnetic materials asdescribed in copending Miller US. patent application Ser. No. 699,030,filed Jan. 19, 1968, entitled Metal-Shell Carrier Particles. Theferromagnetic core can vary in size and shape with useful results beingobtained with core sizes of from about 0.045 to about 0.0015 inchaverage diameter. Particularly useful results are obtained with coresizes of from about 0.023 to about 0.005 inch average diameter. The sizeof the core particles used will, of course, depend upon several factorssuch as the type of images ultimately developed, desired thickness ofthe carboxylated polymer coating etc.

The carboxyl content of the polymers useful in the practice of thisinvention is generally not suificiently high to make the polymerwater-soluble in the free acid form. The carboxyl content is, however,sufiicient to produce solubility when the pH is raised to a value ofabove about 4 to 6 by addition of alkali. Carboxyl contents of about 2to about 15 milliequivalents per gram based on the dry weight of 1 g. ofthe film-forming polymer work satisfactorily. Particularly usefulcoatings can be made with polymers having carboxyl contents varyingbetween about 4 and about 12 milliequivalents. The carboxyl content canbe determined using standard procedures such as titration with a 1 Nsolution of sodium hydroxide using a 1 g. portion of the polymer.

Typical carboxylated polymers useful in this invention include thefollowing, among others: carboxy esterlactone resin as described in U.S.Pat. No. 3,007,901; methyl methacrylate-methacrylic acid 1:1 copolymer;butyl methacrylate-methacrylic acid 1:1 copolymer; styrene-maleic acid(approx. 1:1) copolymer; methylvinyl ether-maleic acid 1:1 copolymer;carboxyester lactone resin prepared as described in US. Pat. No.3,260,706; etc.

The concentration of carboxylated polymer required to inhibitsubstantially all scum formation is as little as about 0.1% of theWeight of the carrier particles coated. High concentrations of up toabout or even higher can be used; however, little further improvement inthe scum-inhibiting effectiveness is realized with higherconcentrations. Nevertheless, the higher concentrations are oftendesirable for purposes other than simply to reduce scum formation. Forexample, as the concentration of resin is increased, the resiliency ofthe carrier particle is enhanced and thus its tendency to abrade thephotoconductive surface is reduced. In general, the preferred range ofpolymer concentration is from about 0.5 to about 2.5% by weight based onthe weight of the core material coated.

In accordance with this invention, an aqueous solution of the polymer ata pH of from about 4 to about 9, can be used. Coating can then becarried out by forming a polymer solution which is sufficiently diluteto readily and uniformly wet all of the uncoated metal carrierparticles. The polymer solution can be added dropwise, as a thin streamor it can be sprayed over the metal carrier power as the powder isturned over or otherwise agitated. In this manner a uniform coating canbe achieved on each particle. Of course, other methods of applicationwhich result in the formation of a thin, continuous, relatively uniformlayer of the aqueous polymer on the carrier particle can also be used.After coating, all solvent is then removed by drying, for example, withforced warm air, while continually mixing or stirring the particles soas to expose all surfaces of the carrier to the drying air.

The scum-preventive coating which is laid down in accordance with thisinvention is of a complex nature. During the drying operation theresin-coated carrier becomes warm as a result of an exothermic reaction.In addition, the color changes gradually from the gray color of ironcarrier particles to a much warmer tone, frequently a coffee-browncolor. The change in color and tthe heat liberated indicate a surfaceoxidation of the particles takes place during the drying operation. Theresulting iron carrier particles thus have a mixed coating of iron oxideand carboxylated resin and/ or the iron salt of the resin. Particleshaving this mixed coating exhibit a substantial increase in surfaceresistance compared to uncoated iron and even when compared with ironcoated with comparable quantities of more highly insulating resins thanthe present carboxylated polymers. For example, iron particles coatedwith 0.5% by weight of a carboxyester lactone resin in accordance withthis invention show a resistance of 450,000 ohms; whereas iron particlescoated with 2.5% by weight of a polyvinyl butyral resin show aresistance of 75,000 ohms.

For purposes of comparison, the resistance of the carrier particles ismeasured in a standard resistance test. The standard test is conductedeach time using a 15 g. quantity of carrier particles. The particles areplaced on a stationary magnet where they are held in the form of abrush. The resistance of the brush is then measured between electrodeshaving a surface area of about 1 square inch. The resistance of theparticles in the magnetic brush 4 is measured with these electrodespositioned about inch apart.

The substantial increase in surface resistance of the present coatedparticles is believed to be attributable mainly to the presence of theiron oxide and/or any iron salts formed with the polymer overcoat. Thecarboxylated polymers by themselves contribute very little to this largeincrease in resistance.

In addition, it has been found that the scum-preventive properties ofthe carboxylated polymer coated carrier particles of the invention aregreater than those having an oxide surface prepared by other means, suchas by heat or other treatment. Furthermore, the present coated particleshave a reduced tendency to scum in comparison with particles coated withcomparable weights of such materials as cellulose acetate butyrate,polyvinyl butyral, polyvinyl acetate, polyethylene and also whencompared with carriers bearing a combination of such a non-carboxylatedresin coating over an underlayer of oxide prepared, for example, byheating the iron powder in air.

The exact reason for the greatly improved resistance to scumming whichthe present particles exhibit is not entirely known. One hypothesis isthat the presence of the carboxyl group in the polymer overcoat resultsin a hy drophilic surface being presented to the normally hydrophobictoner granules. Thus, the only force tending to attract the granules tothe carrier is the triboelectric force between the two materials.Consequently, when the toner granules, carried on the coated particles,are presented to an electrostatic image, the granules are readilyattracted to the image leaving a clean magnetic brush. However, withnon-hydrophilic carrier coatings, there is another attraction betweenthe carrier and the toner in addition to the normal triboelectricattraction. The additional attraction results in the magnetic brushsmearing the hydrophobic toner against the photoconductor. Thisattraction is believed in part to be caused by localized frictionalheating which results in softening of the toner granules. The softenedtoner then appears to act much the same as greasy lithographic ink doeswhen it is attracted to the ink receptive portion of a lithographic; thetoner is similarly attracted to the surface of the prior non-hydrophiliccarrier particles. Thus, there is a greater tendency for toner to adhereto non-hydrophilic carrier particles when used in magnetic-brushdevelopment. Because of this increased attraction, toner granules becomesomewhat rigidly attached to the carrier particles and cannot undergorotational movement. The rigidly held toner is thus continually rubbedagainst the photoconductor. This continual rubbing eventually results inthe deposition of a scum on the photoconductive surface. However, asmentioned above, the carboxylated polymer coating of the presentinvention repels the softened toner so that it does not adhere butremains free to roll against the carrier and/or the photoconductorsurface.

Electroscopic developer compositions can be prepared by mixing thepresent scum-retardent carriers with a suitable electroscopic tonermaterial. The toner used with the carrier particles of this inventioncan be selected from a Wide variety of materials to give desiredphysical properties to the developed image and the proper triboelectricrelationship to match the carrier particles used. Generally any of thetoner powders known in the art are suitable for mixing with the carrierparticles of this invention to form a developer composition. When thepowder toner selected is utilized with ferromagnetic carrier particlesin a magnetic-brush development arrangement, the toner clings to thecarrier by triboelectric attraction. The carrier particles acquire acharge of one polarity and the toner acquires a charge of the oppositepolarity. Thus, if the carrier is mixed with a resin toner which ishigher in the triboelectric series, the toner normally acquires apositive charge and the carrier a negative charge.

Toner powders suitable for use in this invention are typically preparedby micronizing a resinous material and mixing with a coloring materialsuch as a pigment or a dye. The mixture is then ball milled for severalhours and heated so that the resin flows and encases the coloringmaterial. The mass is cooled, broken into small chunks and micronizedagain. After this procedure the toner powder particles usually range insize from about 0.5 to about 25 with an average size of about 2 to about15 The resin material used in preparing the toner can be selected from awide variety of materials, including natural resins, modified naturalresins and synthetic resins. Exemplary of useful natural resins arebalsam resins, colophony and shellac. Exemplary of suitable modifiednatural resins are colophony-modified phenol resins and other resinslisted below with a large proportion of colophony. Suitable syntheticresins are all synthetic resins known to be useful for toner purposes,for example polymers, such as vinyl polymers including polyvinylchloride, polyvinylidene chloride, polyvinyl acetate, polyvinyl acetals,polyvinyl ether and polyacrylic and polymethacrylic esters; polystyreneand substituted polystyrenes or polycondensates, e.g. polyesters, suchas phthalate resin, terephthalic adn isophthalic polyesters, maleinateresin and colophony-mixed esters of higher alcohols; phenol-formaldehyderesins, including colophony-modified phenolformaldehyde condensates,aldehyde resins, ketone resins, polyamides and polyadducts, e.g.polyurethanes. Moreover polyolefins, such as various polyethylenes,polypropylenes, polyisobutylenes and chlorinated rubber are suitable.Additional toner materials which are useful are disclosed in thefollowing US. patents: 2,917,460; Re. 25,136; 2,788,288; 2,638,416;2,618,552 and 2,659,670.

The coloring material additives useful in suitable toners are preferablydyestuffs and colored pigments. These materials serve to color the tonerand thus render it more visible. In addition, they sometimes affect, inknown manner, the polarity of the toner. In principle, virtually all ofthe compounds mentioned in the Color Index, vols. I and II, 2nd ed.,1956, can be used as colorants. Included among the vast numbers ofsuitable colorants would be such materials as Nigrosin Spirit soluble(CI. 50415), Hansa Yellow G (C.-I. 11680), Chromogen Black ETOO (CI.14645), Rhodamine B (CI. 45170), Solvent Black 3 (CI. 26150), Fuchsine N(01. 42510), C.I. Basic Blue 9 (01.52015) etc.

Unless otherwise indicated, the carboxyl content of the polymers of thisinvention is determined by direct titration in 1:1 acetone-water (200ml./ g. of dry polymer) with N/ 2 NaOH to a phenolphthalein endpoint.From this determination, the total free carboxyl groups present can beexpressed as the number of milliliters of N NaOH solution/ gram of dryresin or the carboxyl content can be expressed in milliequivalents of NNaOH/gram of dry polymer.

The following examples are included for a further understanding of theinvention.

EXAMPLE 1 A 2,000 g. quantity of carrier material which comprises bitsof reduced iron having a particle size such that it will pass a 60-meshscreen but will be retained by a 120-mesh screen (Glidden 388 Plast-Ironcarrier), is divided into five portions of 400 g. each. One portion isset aside without further treatment to serve as a control. A secondportion is provided with a thin, surface layer of iron oxide by heatingWhile stirring in a porcelain crucible for 10 minutes at 300 C. Theinitial gray color of the iron changed to purplish black and theresistance as measured in the standard test referred to above changedfrom 2,300 ohms before heating to 84,000 ohms after heating. The otherthree 400 g. quantities are then coated, respectively, with /2 and 2percent by weight of a film-forming carboxyester-lactone resin preparedin accordance with Example 6 of US. Pat. No. 3,007,901 and having acarboxyl content of about milliequivalents of N NaOH/ gram of drypolymer. To accomplish this, a percent 6 solution of the lactone isprepared by dissolving the resin in water with the aid of sodiumhydroxide to produce a solution having a pH of 4.5. Next, three separateportions of the solution, measuring 6.25, 25 and ml., are each dilutedwith cc. of water and each diluted portion is sprayed onto one of theremaining 400 g. quantities of carrier. Each batch is then continuallymixed while drying in a current of air at room temperature. As thepowder dries it begins to warm up to temperatures of about 50 C. At thesame time the coated iron gradually changes to a coffee-brown color.After the powders have dried com pletely and cooled to room temperature,the five samples are tested for abrasiveness and scumming tendency. Theapparatus for testing comprises a cylindrical aluminum tube arranged torotate axially in a horizontal position about a fixed permanent magnet.The permanent magnet has its poles oriented such that when ferromagneticparticles are present, a magnetic brush is formed on the top of thecylinder. In the test procedure this mechanical magnetic brush is runfor 30 minutes in contact with a photoconductive layer comprising anorganic photoconductor in a resin binder. The unmodified carrier(control) and the carrier having the heat-treated oxide surface leaveappreciable scum and abrasion markings on the photoconductor surface.However, no scum is observed with any of the resin-coated carriers andthe abrasion observed is less in all cases. In addition, it appears thatabrasion decreases with increasing concentration of the resin coat. Thefive batches of carrier are then used to make separate triboelectricmixtures with 3 percent by weight of black toner having an averageparticle size of about 8p. and comprising carbon-black colorant in astyrene polymer binder. The toner charges positively on the ironcarrier. The resulting developers are tested in the same manner as thecarriers alone are tested. The control developer causes abrasions andscum although not as severe as in the test without any toner. Similarresults regarding abrasion and scum are observed with the developercontaining the oxidized carrier. None of the three developersrepresenting the resin-coated carriers of the invention produce anyscum. As in the tests with the carrier alone, the abrasion is less thanthat noted with the uncoated carrier and the oxidized carrier. Theabrasion of the resin-coated carrier varies inversely with theconcentration of resin. The five developers are next used to developnegative electrostatic images on organic-photoconductor xerographiclayers of the type used in the scumming and abrasion tests. Images areobtained in all cases. However, the three developers made with theresin-coated carriers develop the image more readily and produce copieshaving better image discrimination with less undesirable background.

EXAMPLE 2 Repeating the procedure of Example 1, six 500 g. quantities ofthe same iron carrier are each coated with a 0.25 percent concentrationof one of the six carboxylated polymers of Table I. The coatings aremade using 12.5 ml. of a 10 percent aqueous solution of each of thepolymers, diluted to 100 ml. with distilled water. Three additional 500g. quantities of iron were each coated with /1 percent of resin from a 1percent solution in dichloromethane of one of the followingnoncarboxylated polymers: cellulose acetate butyrate, polyvinyl butyraland polyvinyl acetate. A fourth quantity of 500 g. of iron is coatedwith percent polyethylene from a 2 percent solution in hot cyclohexane.Each of the ten quantities are individually dried with forced air atroom temperature while stirring. The six quantities of carrier coatedwith a carboxylated polymer warm appreciably during the drying operationand turn brown. The other four quantities drop below room temperatureduring the drying, as a result of evaporative cooling. In addition, nocolor change is observed in the particles coated with thenoncarboxylated polymers. The scum and abrasion tests of Example 1 arerepeated with all ten coated materials and with a control of a 500 g.quantity of uncoated iron. A reduction of abrasion of thephotoconductive surface of about the same order of magnitude is notedwith all ten samples when compared with the uncoated control iron. Allten resincoated samples produce a reduction in the degree of scumming,both in the tests with the carrier alone and in tests using a developercomprising 4 percent by weight of a toner consisting of a nigrosinecolorant in a styrene butylmethacrylate copolymer binder. However, thefour carriers coated with noncarboxylated polymers reduced this scummingonly partially; whereas, scum is essentially completely inhibited by theuse of the six carboxylated-resincoated carriers.

TABLE I Carboxyl content in milliequivalents of N Carboxylated polymerNaOH/g. pH 1 I Carboxy ester-lactone resin prepared as described inMinsk U.S. Patent 3,007,- 901 2 5. 4. 5 II Methylmethacrylate-methacrylic acid 1:1 copolyrncr 5. 8 6. 5 III Butylmethacrylatemethacrylic acid 1:1

eopolymer 4. 7 7. 5 IV Styrene-maleic acid (approx. 121) copolymer(inherent viscosity of the styrene-maleicanhydride copolymer precursor,0.22) 2 0. 0 7. 9 V Methylvinyl ether-maleic acid 1:1 00- polymer(Gantrez AN-119 from General Analine and Film Company) 11. 5 7. 2 VICarboxy ester-lactone resin prepared as described in Minsk-Abel U.S.Patent 3,260,706 2 5. 0 6. 6

1 Adjusted in 10 percent aqueous solution with NaOH. 2 Approximately.

Carboxylated polymer No. I is a mixed alkyl ester and carboxy esterlactone prepared by reaction of glycolic acid and n-hexyl alcohol withan interpolymer of vinyl acetate and maleic anhydride in accordance withExample 6 of US. Pat. No. 3,007,901. This polymer is made by mixing 1390ml. of dioxane, 940 ml. of 70% glycolic acid, 450 ml. n-hexanol and 300g. of vinyl acetate-maleic anhydride heteropolymer (inherentviscosity=0.2l). The suspension is stirred until smooth and 108 ml.concentrated sulfuric acid is added. The resultant dope is stirred andheated for several hours, cooled and diluted with acetone. The diluteddope is then poured into distilled water, stirred and washed again indistilled water until free of mineral acid. Finally the polymer is driedat room temperature. Polymer No. VI above is prepared in accordance withExample 1 of US. Pat. No. 3,260,706 by stirring for 1 to 2 hours amixture of 200 grams of vinyl acetate-maleic anhydride interpolymer(inherent viscosity=0.29), 600 ml. of distilled water and 75 ml. ofn-butylalcohol. Next 68 ml. of concentrated sulfuric acid are added andthe mass is heated for about 20 hours. The solid formed is separatedfrom the liquid and kneaded in a mill with repeated changes of water.

The polymer is then oven dried overnight at 40 C.

This invention has been described in considerable detail with referenceto certain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention as described hereinabove and as defined in the appendedclaims.

I claim:

1. In a developer composition for use in the develop- 8 ment ofelectrostatic charge patterns and comprising a physical mixture ofmagnetically attractable carrier particles and smaller electroscopictoner particles, the improvement wherein said carrier particles arecomprised of a core of ferromagnetic material overcoated with a thin,continuous layer of a film-forming, alkali-soluble carboxylated polymerhaving a carboxyl content of from about 2 to about 15 milliequivalentsof 1 N NaOH per gram of polymer based on the dry weight of said polymer.

2. A developer composition as described in claim 1 wherein saidcarboxylated polymer is present in an amount of from about 0.1 to about5.0 percent by weight based on the weight of the core.

3. A developer composition as described in claim 1 wherein theferromagnetic core contains a material selected from the groupconsisting of iron, nickel, cobalt and alloys thereof.

4. A developer composition as described in claim 1 wherein the polymeris methyl methacrylate-methacrylic acid copolymer.

5. A developer composition as described in claim 1 wherein the polymeris butyl methacrylate-methacrylic acid copolymer.

6. A developer composition as described in claim 1 wherein the polymeris methylvinyl ether-maleic acid copolymer.

7. A developer composition as described in claim 1 wherein the averagesize of the core particle is from about 0.045 to about 0.0015 inch.

8. A developer composition as described in claim 1 wherein saidcarboxylated polymer is a mixed alkyl ester and carboxy ester lactoneprepared by reaction of glycolic acid and n-hexyl alcohol with aninterpolymer of vinyl acetate and maleic anhydride.

9. An electroscopic developer composition comprising a mixture of tonergranules and carrier particles wherein each carrier particle comprises acore of ferromagnetic material overcoated with a thin, continuous layerof a film-forming, alkali-soluble carboxylated polymer having a carboxylcontent of from about 4 to about 12 milliequivalents of N NaOH per gramof polymer based on the dry weight of said polymer.

10. An electroscopic developer composition as in claim 9 wherein theferromagnetic material is selected from the group consisting of iron andiron alloy and wherein the polymer is present on each particle in anamount of from about 0.5 to about 2.5 percent by weight based on theweight of the core.

References Cited UNITED STATES PATENTS 3,121,642 2/1964 Bishup 117-1003,054,751 9/1962 Blake et a1. 1l7100 2,890,968 6/1959 Giamo 25262.12,874,063 2/1959 Greig 117-l7.5

GEORGE F. LESMES, Primary Examiner J. P. BRAMMER, Asistant Examiner US.Cl. X.R. 11717.5,

" UNITED STATES PATENT OFFICE 56g CERTIFICATE OF CORRECTION Patent No.3,547,822 Dated December ,15, 1970 Inventor(s) Howard A- i ler It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 8, line 42, "of N NaOH" should read "of 1N NaOH".

Signed and sealed this ZLLth day of August 1971.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. WILLIAM E. SCHUYLER, JR. Attesting OfficerCommissioner of Patents

